The present invention relates to the field of electronics, and in particular to low-dropout voltage regulators.
Low-dropout voltage regulators have been used for battery applications, e.g., in cellular phones, etc. FIG. 1 shows a conventional low-dropout regulator (LDO) 10 that is connected to a load 20. LDO 10 includes an op-amp 12, a PMOS transistor M1, resistors R1 and R2, and a reference voltage supply Vref. Load 20 includes a resistive load RL and a capacitive load CL. A very serious problem associated with this circuit is that it is not stable for all capacitive loads (CL). Known solutions can stabilize this circuit for values of CL larger than approximately 1 uF. Another restriction associated with this circuit is that the capacitor must have a low and very well-defined equivalent series resistance (ESR), which is inherent in any capacitive loads. Examples of such LDO""s are Maxim""s MAX8863, Telcom""s TC1072, Linear""s LT1121, which are available from Maxim Integrated Products, Inc., Telcom Semiconductors, Inc. and Linear Technology Corporation, respectively.
Therefore, there is a need for an improved low-dropout voltage regulator that is suitable for all capacitive loads and that removes the ESR restrictions on the loads.
The present invention provides an LDO that is stable for all capacitive loads. Because the LDO is stable for all capacitive loads, the ESR can no longer affect the equivalent value of the combination of the ESR and the capacitive load. Thus, the invention also effectively removes the ESR restrictions on the loads.
According to the present invention, a low dropout voltage regulator is provided. The regulator comprises a switching element (e.g., a transistor) having first terminal for receiving an input signal, a second terminal for providing an output signal and a control terminal; a control circuit, operably coupled to the switching element, that is configured to control the switching element; and a compensation circuit having a first segment connected between the first and control terminals of the switching element and a second segment connected between the control and second terminals of the switching element.
According to the invention, the first segment of the compensation circuit includes a first resistor and the second segment of the compensation circuit includes a RC circuit. In one embodiment of the invention, the RC circuit includes a second resistor and a capacitor connected to each other in series. In another embodiment of the invention, the RC circuit includes a distributed RC network having a plurality of resistors and capacitors.
According to the invention, the control circuit includes an operational amplifier having an output terminal connected to the control terminal of the switching element, and a pair of resistors connected in series between the second terminal of the switching element and a first voltage reference level. The amplifier of the control circuit has a positive terminal connected between the pair of resistors and a negative terminal connected to a second voltage reference level.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.